The reception and processing of broadcast RF (radio frequency) signals are known in the art. Historically, the reception and processing of RF signals was performed using analog circuitry in which a number of analog components received the signals, and passed the information contained in the amplitude and phase portions of the signals along for further processing and output. More recently, the advent of digital circuitry has allowed designers to create communication devices which employ digital signal processing in the processing of the RF signals.
A conventional digital receiver 10 utilized in radio communication systems is illustrated in FIG. 1. The conventional digital receiver 10 receives a broadcast input signal 14 sent from a radio communication system 12 in an analog format using an antenna 16. A (radio frequency) RF interface 18 then amplifies the broadcast input signal 14, passes the frequency component of the broadcast input signal 14, and remodulates the desired frequency component to an (intermediate frequency) IF signal 20. An analog mixer 22 then mixes down to baseband the IF signal 20 using a reference frequency 26, such as the output of a voltage controlled oscillator, to produce a baseband signal 24 . An analog to digital converter 28 receives the baseband signal 24 and converts it from an analog signal to a digital signal 30 which includes an in-phase signal and a quadrature signal. The digital signal 30 is then digitally filtered by a filter 32 to improve the selectivity of the receiver and reduce the adjacent channel interference. The filtered signal 34 is then decimated to a lower sampling frequency, the decimation step consisting of down-sampling with a down-sampler 36; and low pass filtering with a low pass filter 40 to a new sampling rate. Downsampling comprises selectively discarding input data and adjusting the output data such that the transmission rate of the output data is at a different clock rate than the input data clock rate. Once the signal 42 is sampled at a lower rate, it can be further processed digitally to remove induced noise and to retrieve the signal that was encoded by the radio communication system 12.
Due to frequency fluctuations between the carrier frequency and the local oscillator frequency, continual adjustment of a frequency difference between the two frequencies is required to avoid errors in the retrieval of the signal. This procedure of adaptively adjusting the local oscillator frequency to track the carrier frequency is known as automatic frequency control (AFC). The frequency shifts in the signal due to drifts in the local oscillator frequency in the digital receiver's front end (such as RF interface 18) and other noise factors make AFC a challenge to implement.
One conventional method of AFC is shown in FIG. 1, comprising a demodulator 44, a (digital signal processor) DSP 48, a digital to analog converter 52 and a (voltage controlled oscillator) VCO 56. The demodulator 44 demodulates the signal to produce a conventional FM demodulated signal 46. FM information is then extracted from the conventional FM demodulated signal 46. The DSP 48 determines a frequency offset and compensation value, outputs the digital frequency compensation value 50, which is converted to an analog frequency compensation value 54 by the digital to analog converter 52. This analog frequency compensation value 54 is used to vary the frequency of the VCO 56 and the output of the VCO 56 is the reference frequency 26. This technique requires the use of a high precision digital to analog converter 52. In most cases the high precision digital to analog converter 52, which is essential to accurate frequency compensation, can be difficult and expensive to realize.
A second conventional method of AFC eliminates the digital to analog converter 52 and performs the AFC using software within the DSP 48. In this method the DSP 48 mixes the signal to the appropriate frequency by using a software complex multiplier; and additionally performs the a numerically controlled oscillator function to generate a complex exponential. FIG. 2 is a block diagram illustrating a second conventional digital receiver 60 utilizing this method.
In FIG. 2, the second conventional digital receiver 60 receives a broadcast input signal 14 sent from a radio communication system 12 using an antenna 16. The broadcast input signal 14 is processed through the RF interface 18, the analog mixer 22, the analog to digital converter 28, the filter 32, and the down-sampler 36 similarly to previously described for the conventional digital receiver 10. In some scenarios, the analog mixer coarsely adjusts the signal using the AFC computations as the reference 21.
In the second conventional AFC method shown in FIG. 2, the output of the down-sampler 36, the downsampled signal 64, is received by a digital mixer 62, which then quadrature mixes down the downsampled signal 64 using an AFC reference frequency 82. The signal continues to be processed through a filter 40, and a demodulator 44 as previously described for the conventional digital receiver 10 of FIG.1.
In this second conventional method, the output of the demodulator 44, the demodulated signal 70, is fed into a complex multiplier consisting of an integrator 72, and a (numerically controlled oscillator) NCO 80. The integrator 72 receives a plurality of demodulated signals 70, and performs a successive accumulation function including all past values of the demodulated signals 70. The NCO 80 receives the output of the integrator 72, the integrated signal 76, and generates a complex value representation of the phase of this integrated signal 76. The output of the NCO 80 is the AFC reference signal 82 which is mixed with the downsampled signal 64 in the digital mixer 62.
The implementation of this second conventional method of automatic frequency control using DSP software is computationally complex and may require a significant amount of processing power.
The two conventional apparatus and techniques for automatic frequency control described above are representative of the design approaches currently utilized. Analysis of these two apparatus indicates that there exists a need in the art for an apparatus that efficiently, with minimal circuit components and minimal power consumption, performs automatic frequency control.